For ophthalmic laser procedures wherein eye tissue is to be photdisrupted or ablated, it is extremely important for the laser beam to be properly focused to a specific focal spot in the tissue that is to be affected. Also, it is extremely important that the focal spot have good definition. To do all of this, it is necessary for the laser beam to be as free from aberrations as possible. Considerations here include the eye itself, as well as the laser system. In particular, for ophthalmic laser procedures involving the cornea, it happens that the spherical geometry of the cornea introduces aberrations on its own which are separate and independent of the laser system being used. Importantly, these corneal induced aberrations distort the definition of the focal spot of the laser beam in the cornea. In order to improve this situation, these aberrations need to be eliminated or significantly minimized.
Due to the spherical geometry of the anterior surface of the cornea, two types of aberrations are of particular importance. These are: spherical aberration (which relates to points on the optical axis of the laser beam), and coma (which relates to points that are off-axis). Spherical aberration and coma are similar to each other in that they both arise from a failure to image or focus rays at the same point. Coma differs from spherical aberration, however, in that a point object is imaged not as a circle but as a comet-shaped figure (whence the term "coma"). Nevertheless, in both cases, there is a loss of definition at the focal spot.
By definition, an aplanatic lens is one which is free from both spherical aberration and coma. Still, because an interface between different media is involved, the sine condition must be considered. It then follows that aplanatic refraction results under conditions in which there is no spherical aberration or coma, and in which the sine condition is satisfied. Mathematically, the sine condition is satisfied when: EQU n.sub.1 I.sub.1 sin.alpha..sub.1 =n.sub.2 I.sub.2 sin.alpha..sub.2
where n.sub.1 and n.sub.2 are the refractive indices of the media on the laser source and focal spot sides of a media interface respectively, I.sub.1 and I.sub.2 are the linear dimensions of the laser source and focal spot, and .alpha..sub.1 and .alpha..sub.2 are the angles made with the principal axis by the conjugate portions of a ray passing between the laser source and the focal spot through the media interface.
As recognized by the present invention, aplanatic refraction at the anterior surface of the cornea can be effectively accomplished by flattening the anterior surface. With such a reconfiguration of the cornea, as a laser beam enters the cornea the sine condition will be satisfied and, importantly, the laser beam will be free of aberrations (other than chromatic) which would otherwise result from the spherical geometry of the cornea's anterior surface.
In light of the above, it is an object of the present invention to provide a disposable aplanatic lens which will reconfigure the cornea for surgical laser procedures with a disposable aplanatic configuration. Yet another object of the present invention is to provide a disposable aplanatic lens which will stabilize and maintain a proper orientation of the eye during ophthalmic laser surgery. Still another object of the present invention is to provide a disposable aplanatic lens which will beneficially reduce intraocular pressure during ophthalmic laser surgery. Another object of the present invention is to provide a disposable aplanatic lens which is easy to use, relatively simple to manufacture and comparatively cost effective.